The invention relates to an on-board electrical system for a motor vehicle. The on-board electrical system includes a vehicle battery, an electrical load, and an energy storage device having a positive and a negative terminal. Switching arrangement are provided which connect in a first switching state the positive terminal of the energy storage device to the vehicle battery and the negative terminal of the energy storage device to the electrical load and which can be brought into at least one additional switching state that is different from the first switching state. A control device can switch the switching arrangement between the first switching state and the additional switching state. The invention also relates to a motor vehicle with on-board electrical system of this type and a method for operating an on-board electrical system of a motor vehicle.
Electrical loads providing functions relevant for the safety of vehicle occupants are nowadays increasingly used in motor vehicles. Such safety-relevant loads typically consume a large amount of power, but require overall little energy. Such electrical loads are used as replacement for mechanical or hydraulic systems to lower the fuel consumption while improving functionality. The loads are primarily electric motors employed, for example, for the steering system, a parking lock for a parking brake, or in a braking system (for example, the ESP, the “Electronic Stability Program”). The loads may also be electronic devices, such as communication devices (such as mobile phones) for making an emergency call. On one hand, the electrical supply for these loads must always be assured; on the other hand, the current consumption of the electrical load is not constant, since they are switched on only when needed, and may therefore impair the operation of sensitive load of the on-board electrical system.
The on-board electrical system of a motor vehicle includes in the simplest case a vehicle battery, a generator and a plurality of (sensitive) loads. With the engine running, the generator provides a voltage for supplying the loads and charging the vehicle battery. The output power from the generator can also be adjusted by a controller to the respective instantaneous power requirement of the electrical loads. However, the novel electrical (safety-relevant) loads stress the on-board electrical system with high pulsed currents. Currently used generators are too slow to provide these pulsed currents or to rapidly increase or decrease the voltage. The on-board system voltage is therefore mainly stabilized by the vehicle battery, with the stability of the on-board system voltage being determined by the internal resistance of the vehicle battery. Under high pulsed currents, the on-board system voltage can collapse by several volts, thus temporarily disturbing the function of sensitive loads. Such behavior is particularly problematic with the new start-/stop-systems employed in motor vehicles.
Different systems have been developed in the past to reduce voltage drops and to protect sensitive loads. Most of these systems use double-layer capacitors or batteries as an additional energy storage device for the on-board electrical system. In many existing systems, the additional energy storage device is connected in parallel with the vehicle battery; this parallel connection reduces the total impedance and hence also the voltage drop of the on-board system. Such electrical systems are known, for example, from the documents DE 10 2005 015 995 A1 and DE 10 2007 026 164 A1.
An additional energy storage device can also be connected via an intermediate DC-DC converter, as described in the publications WO 02/066293 A1 and DE 198 59 036 A1.
It is known to supply the sensitive loads directly from the additional energy storage device, thereby decoupling the sensitive loads from the high-power loads. Such an approach is described, for example, in Robert Bosch GmbH, “Autoelektrik, Autoelektronik, Systeme and Komponenten” (Electric Car Circuitry, Car Electronics, Systems and Components), 4th Edition, Vieweg Verlag, Wiesbaden, ISBN 3-528-13872-6, page 16, FIG. 7.
Currently, there is a trend to connect an additional energy storage device, such as a double layer capacitor, in series with the vehicle battery. Such on-board electrical system is known from the document Continental, ELKS 2008-“Elektrische Leistungsbordnetze and Komponenten von Straβenfahrzeugen” (Electrical on-board power systems and components of road vehicles), Contributions to the first Symposium with the same name on Oct. 8 and 9, 2008, TU Braunschweig, ISBN: 978-3-937655-17-8, page 90. In this on-board electrical system, the voltage drop at an engine start is compensated by connecting a double layer capacitor in series with the vehicle battery. In an ideal situation, this series circuit may provide a voltage greater than the battery voltage to the load, when the starter is not actuated. A low voltage is thereby compensated. A series connection of a car battery and a double layer capacitor is also known from the published application DE 10 2005 042 154 A1.
The present state of the art is generally concerned with the problem of compensating voltage drops in the on-board electrical system. It is a particular challenge to not only compensate the voltage drops without adding complexity, in particular without using an expensive DC-DC converter, but to also prevent overvoltages in the on-board electrical system. One remedy is an on-board electrical system of the type disclosed in DE 10 2009 024 374 A which was published after the filing date of the present application. This on-board electrical system includes switching arrangement configured to connect the positive terminal of an energy storage device to the vehicle battery and the negative terminal of this energy storage device to an electrical load. Thus, a voltage dependent on the electrical loads may be supplied to the electrical load, which is lower than the electrical voltage applied to the vehicle battery, which may be a battery voltage or a generator voltage of a generator connected in parallel with the battery. This lower voltage is attained with the additional energy storage device, for example with a double layer capacitor (also known under the name of Super-Cap), i.e. without using an expensive DC-DC converter. With this known on-board electrical system, an overvoltage at the electrical load can advantageously be compensated, especially when a generator is connected in parallel with the battery. Such an overvoltage may arise, for example, when a high-power electrical load connected in parallel with the vehicle battery is switched off. This produces a voltage in the on-board electrical system, which is larger than the voltage produced by the generator in normal operation. When such an overvoltage is produced, the generator alone is not able to compensate the overvoltage fast enough.
When a reliable operation of the sensitive loads is guaranteed by the solution proposed in DE 10 2009 024 374 A, the attention shifts to a reliably supply of electrical energy to those loads whose functions are relevant for the safety of vehicle occupants. Such loads require at least two separate or redundant sources of energy, which can independently supply electric power. It must be ensured that these loads are supplied with electrical energy even when one power source fails, namely with electrical energy from the respective other energy source. An on-board electrical system with two separate systems—a primary system and a secondary system—for redundant supply of electrical energy to safety-relevant loads is already known, for example, from the published patent DE 10 2006 010 713 B4. The safety-relevant load can be supplied with electrical energy, as needed, from a vehicle battery or from a double layer capacitor. However, overvoltages occurring in this on-board electrical system cannot be compensated.